There are a number of different technologies known for the creation of stereoscopic 3d images for cinema projection applications. One such technology described, for example, in US Patent No. 2006/0291053A1 dated 14 Jun., 2006 entitled “Achromatic Polarization Switches”, uses a polarization modulator comprising one or more liquid crystal elements stacked together and placed directly in front of a projector. The projector is designed to generate a succession of alternating left-eye and right-eye images at high frequencies, typically 144 Hz. The polarization modulator imparts an optical polarization state to the succession of left-eye and right-eye images.
Furthermore, by synchronization of the polarization modulator together with the succession of images being generated by the projector, the optical polarization states of the left-eye images and right-eye images are arranged so as to be mutually orthogonal. Thereafter, by focusing the images onto a polarization preserving projection screen, the observer is able to view time-multiplexed stereoscopic-3d images via utilisation of passive polarized viewing-goggles.
However, since a linear polarization filter is required to be mounted on the entrance surface of the polarization modulator, approximately 50% of the initially unpolarized incident light generated by a typical cinema projector is absorbed. In addition, the time-multiplexed duty-cycle for both the left-eye images and right-eye images emitted by the projector is only 50%. Therefore, the maximum theoretical optical light efficiency of such a stereoscopic-3d system is limited to being only 50%×50%=25%. Moreover, in practice the overall optical light efficiency of such systems may in fact be as low as typically only 18% due to there being additional optical losses, such as surface reflections, etc. This results in the generation of stereoscopic-3d images that are severely lacking in on-screen image brightness.
In order to improve the overall optical light efficiency of a stereoscopic-3d system, one technology is described, for example, in Patent Application No. PCT/EP2012/064069, dated 18 Jul. 2012, entitled “Device for polarizing a video sequence to be viewed stereoscopically”. This system uses a beam-splitter arrangement placed directly in front of a projector in order to split the initially unpolarized incident light generated by the projector into two separate secondary image beams. The secondary image beams travelling in mutually opposite directions are also both orthogonal to the direction of the original in-coming incident light generated by the projector and said secondary image beams are linearly polarized in mutually orthogonal orientations, for example with the first image beam possessing s polarization and said second image beam possessing p polarization, respectively.
Thereafter, polarization modulators comprising one or more liquid crystal elements stacked together are used to modulate the polarization states of the secondary image beams and are arranged such that at any given instance in time both secondary image beams are imparted with mutually identical optical polarization states. Furthermore, the polarization modulators are synchronized together with the succession of left and right eye images generated by the projector and arranged such that the left and right eye images are optically polarized in mutually orthogonal orientations.
Both secondary image beams are thereafter mutually aligned using reflecting mirrors and focused onto a polarization preserving projection screen, thereby enabling the observer to view time-multiplexed stereoscopic-3d images via utilisation of passive polarized viewing-goggles.
In theory, since 100% of all light initially generated by the projector is reflected towards the projection screen when using the aforementioned prior-art technology, the maximum theoretical optical light efficiency will now be increased to a value of 100%×50%=50%. This is because the time-multiplexed duty-cycle for both the left and right eye images is still only 50%. This creates stereoscopic-3d images with an improved level of on-screen image brightness; however, in practice the overall optical light efficiency of such systems may be typically less than 30% due to the occurrence of other optical losses such as surface reflections.
Another prior-art system which further increases the optical light efficiency of a stereoscopic-3d projection system is described, for example, in U.S. Pat. No. 7,959,296 B2, dated 27 Dec., 2010, and entitled “Combining P and S rays for bright stereoscopic projection”. With this system two individual projectors are placed together with the first projector generating only the left-eye images and the second projector generating only the right-eye images. It will be understood by one skilled in the art that in such arrangement the time-multiplexed duty-cycle for both the left and right eye images will now be increased to almost 100%.
Furthermore, when used together with a beam-splitting arrangement placed in front of each individual projector which, in theory diverts and reflects 100% of all incident light generated by each individual projector towards the projection screen, the overall theoretical optical light efficiency of such system will then be increased to 100%×100%=100%. However, in practice the optical light efficiency of such system is only approximately 50% due to the occurrence of optical losses such as surface reflections.
In aforementioned U.S. Pat. No. 7,959,296 B2, it is described how a single beam-splitting element, such as a planar wire-grid polarizer, can be placed directly in front of each individual projector to split the incident light beam emitted by each individual projector into two separate image beams. They include one primary beam with a first optical linear polarization state travelling in the same direction as the original incident light beam, and one secondary beam with a second optical linear polarization state travelling in an orthogonal direction to said original incident light beam. Thereafter, a mirror is used to reflect said secondary beam towards a projection screen and both primary and secondary image beams are thereby arranged so as to mutually overlap on the surface of the projection screen.
However, it will be understood by one skilled in the art that such an arrangement results in there being a relatively large difference in the optical path lengths for the primary and secondary beams. This requires either introducing an optical path length compensation element in the path of the primary beam, such as a telephoto lens pair, or using a deformable-mirror to reflect the secondary beam towards the projection-screen which introduces a high-level of optical image convergence, or using a combination of both methods thereof.
However, the use of a telephoto-lens pair for the primary-beam in order to compensate for the optical path length difference between said primary and secondary beams will reduce the overall optical light efficiency of the system due to the occurrence of optical losses such as surface reflections. In addition to this, it will be understood by one skilled in the art that the utilisation of a deformable mirror in order to generate a high level of image convergence for the secondary beam will result in the design of a relatively complex and expensive system.
It is also specifically described in the aforementioned U.S. Pat. No. 7,959,296 B2 how an optical polarization rotator is required for at least one of the primary and secondary beams in order to rotate the optical linear polarization state of at least one of said primary and secondary beams by 90 degrees. However, the incorporation of a polarization rotator will further reduce the overall optical light efficiency due to associated optical losses as well as adding both complexity and expense to the overall system.